Even in the field of pneumatic tires as a typical example of rubber articles, the weight reduction of the tire is recently progressing for contributing to low fuel consumption of a vehicle in view of global environmental protection. This is true even when steel cords are used as a reinforcement for the tire. That is, it is attempted to decrease the amount of steel cords used for reducing total weight of carcass or belt by increasing the strength of the cord while maintaining the strength of the tire.
However, if it is intended to increase the tensile strength per unit section area of a steel filament constituting the steel cord, the weight reduction of the tire is realized, but the resistance to corrosion fatigue is degraded. Hence there is caused a problem that the durability of the cord and hence the durability of the rubber article such as tire or the like are lowered.
That is, the tire may be sometimes subjected to cut failure arriving at the vicinity of the steel cord through protrusions and foreign matter on a road surface or a roadside edge and hence the steel filaments constituting the cord are exposed to corrosion environment through water penetrating into such a cut failure to lower the durability of the tire. This phenomenon becomes conspicuous as the tensile strength of the cord becomes higher.
As a countermeasure, there is known a method wherein the forming ratio of the steel filament is made large to form a space between the steel filaments in case of steel cord having a single twisted construction, or the number of steel filaments constituting a sheath is decreased to form a space between the steel filaments in case of steel cord having a layer twisting construction, whereby the steel cord is rendered into a so-called rubber-penetration structure provided with the space capable of inserting rubber into the inside of the cord to prevent the penetration of water into the inside of the cord and hence control the corrosion of the steel filaments.
Even if rubber penetration is sufficient, however, the corrosion of the steel filament may be progressed by water included in rubber, so that the resistance to corrosion fatigue is not necessarily improved even by the above rubber-penetration structure.
Further, JP-A-57-149578 proposes a method wherein the fatigue resistance is improved by alternately conducting bending-unbending treatments to uniformly disperse residual compression stress over a full peripheral surface of a wire. In the cord having a high tensile strength, however, the elastic limit is high, so that there is a disadvantage that the effect by the bending-unbending treatments is not sufficiently developed.